Step 1: Prepare the Grapefruit. Using a sharp knife, carefully slice off the top and bottom of each grapefruit. Stand one grapefruit on a flat end. Following the curve of the fruit, slice downwards to remove the peel and white pith, exposing the juicy flesh. Once peeled, carefully cut between the membranes to release individual grapefruit segments (supremes) into a bowl. Collect any juice that drips out.
Step 2: Slice the Avocado. Halve the ripe avocados, remove the pits, and scoop out the flesh. Slice the avocado into bite-sized pieces or cubes. Toss gently with a splash of the collected grapefruit juice or a squeeze of fresh lime juice immediately after cutting to prevent browning.
Step 3: Prepare the Red Onion. Thinly slice the red onion. If desired, soak the slices in ice water for 10-15 minutes, then drain and pat dry to mellow their flavor.
Step 4: Whisk the Vinaigrette. In a small bowl, combine the fresh lime juice, honey, olive oil, sea salt, and black pepper. Whisk vigorously until the dressing is well emulsified.
Step 5: Assemble the Salad. In a large mixing bowl, gently combine the grapefruit segments, avocado slices, and red onion. Pour the honey-lime vinaigrette over the salad. Gently toss until all the ingredients are lightly coated.
Step 6: Garnish and Serve. Transfer the salad to a serving platter or individual bowls. Garnish with fresh cilantro or mint leaves, if using. Serve immediately to enjoy its peak freshness.